1. Field of the Invention
The invention relates to a process for cleaning CVD-production rooms for producing polycrystalline silicon. The invention relates to rooms or factory halls in which polycrystalline silicon is produced by means of CVD (chemical vapor deposition).
2. Description of the Related Art
Polycrystalline silicon is usually produced by means of the Siemens process. In this process a reaction gas comprising one or more silicon-containing components and optionally hydrogen is introduced into a reactor comprising substrates heated by direct passage of current, wherein silicon deposits in solid form on the substrates. The silicon-containing components preferably used are silane (SiH4), monochlorosilane(SiH3Cl), dichlorosilane (SiH2Cl2), trichlorosilane (SiHCl3), tetrachlorosilane (SiCl4) or mixtures of the said substances.
The Siemens process is usually carried out in a deposition reactor (also termed “Siemens reactor”). In the most familiar embodiment, the reactor comprises a metallic baseplate and a coolable bell which is mounted on the baseplate, in such a manner that a reaction chamber is formed in the interior of the bell. The baseplate is provided with one or more gas inlet openings and one or more off-gas openings for the departing reaction gases and also with holders with the aid of which the substrates are held in the reaction chamber and supplied with electric current.
A plurality of such Siemens reactors are arranged in one production room. In the process, the facility also comprises a multiplicity of pipelines in order to supply the reactors with the reaction gases, namely silanes and optionally hydrogen, and in order to remove off-gases which are formed in the deposition.
Polycrystalline silicon must satisfy high purity requirements.
The polycrystalline silicon can be contaminated during removal of the rods from the reactor and during the subsequent processing steps.
The majority of the polycrystalline silicon rods produced must be comminuted to fragments, since only fragments can be further processed by the clients of the electronics and solar industries. One exception is formed by polycrystalline silicon rods which are converted directly into monocrystalline silicon rods by means of the float-zone (FZ) process and are then turned into wafers.
The rods are usually mechanically comminuted by means of crushers, which comprise metallic crushing tools. In this process the polycrystalline silicon is contaminated on the surface. Attempts are made to counteract this by various measures, e.g. via crushing tools made of hard metal which are particularly abrasion-resistant and as a result give off fewer metals to the polysilicon. If the polycrystalline silicon is intended for the electronics industry, in addition a wet-chemical cleaning of the fragments is performed, in order to etch away metals from the surface of the polysilicon.
However, the polycrystalline silicon is exposed to environmental influences during the entire processing. This begins with the removal of the rods from the reactor, and relates to the transport processes and to the following processing steps and processing in crushing, cleaning and packaging facilities. This can also lead to contamination.
In particular, removal of the rods from the reactor has been taken to be the critical step in polysilicon fabrication.
After the rods have cooled, the reactor bell is opened and the rods are withdrawn by hand or using special devices, termed removal aids, for further processing, or for interim storage. US 20100043972 A1 discloses a further device for removal of polycrystalline silicon rods, comprising a wall having an inner wall, an outer wall and a multiplicity of connections between inner wall and outer wall, a gap between inner wall and outer wall, an access window in the outer wall, a baseplate, and a multiplicity of contacts on the baseplate, wherein inner wall and outer wall are cylindrical and concentric, and the gap is dimensioned in such a manner as to take up a multiplicity of silicon rods situated on the contacts of the baseplate, wherein the access window is designed in such a manner that access to the silicon rods is made possible. The rods can be withdrawn via the access windows.
Not only storage but also further processing, especially comminution of the rods, classification and packaging of broken pieces, generally proceed under particular environmental conditions in climatically controlled rooms, which prevents contamination of the product.
Between the time point of opening the reactor and up to storage or further processing, the deposited material, however, is exposed to environmental effects, in particular dust particles.
It has been proposed to use what is termed a removal bag during removal of the polysilicon rods and to cover the silicon rod therewith. US 20120175613 A1 discloses a process for producing a polycrystalline silicon piece, consisting of a CVD process for producing a polycrystalline silicon rod by depositing silicon on a filament wire, one end of which is connected to a first electrode and the other end of which is connected to a second electrode, a process for withdrawal of the polycrystalline silicon rod from the reactor and a process for comminuting the silicon rod into silicon pieces, wherein, before the comminution process, at least 70 mm are removed from the electrode end of the polycrystalline silicon rod (shortening process). In a preferred embodiment, the surface of the polycrystalline silicon rod is covered before withdrawal from the reactor with a bag-like part made of polyethylene. The withdrawal itself can proceed by means of a crane or the like. The bag-like part is intended to prevent, during rod removal, metal grains containing nickel, chromium and copper, which after opening of the reactor are released into the working atmosphere, from accumulating on the surface of silicon rods.
In principle, it is also conceivable to carry out the production process in a clean room. US 20020040568 A1 discloses a facility for fabricating semiconductor products, in particular wafers, having an arrangement of fabrication units in at least one clean room having an air supply system, wherein, in the air supply system, the air feed proceeds via the floor of the clean room. A significant advantage of this air supply system is that the circulation of feed air and exhaust air due to gravity is exploited thereby in such a manner that the feed air is provided with a low energy consumption at the sites for processing the semiconductor products.
However, facilities for producing polycrystalline silicon have a multiplicity of Siemens reactors. It must be considered that the deposited rods can be several meters high and weigh several hundred kg. Therefore, such facilities are enormously large. Constructing a hall having dozens of Siemens reactors as a clean room is scarcely conceivable economically.
These problems resulted in the objective of the invention.